1. Field of the Invention
The present invention generally relates to a device having a capacitor, such as an oscillator, and more particularly, to an interdigital capacitor used in a high-frequency circuit and a method for adjusting the same.
2. Description of the Related Art
An MIM (Metal Insulator Metal) capacitor and an interdigital capacitor are known as capacitors used in high-frequency circuits that handle a frequency as high as 1 GHz or over. Usually, these capacitors are incorporated in an MMIC (Monolithic Microwave Integrated Circuit). For example, the MIM is made up of a pair of electrodes formed on a semi-insulating GaAs substrate, and a dielectric member sandwiched between the pair of electrodes. The interdigital capacitor has an interdigital electrode pattern formed on a semi-insulating GaAs substrate.
Generally, the capacitor is required to have a smaller capacitance, as the frequency is higher. For example, the capacitance required in the 40 GHz band is as small as 30 fF. The capacitor having such a small capacitance should be compact. For instance, a capacitance of 30 fF by an MIM capacitor having a per-unit-area capacitance of 0.4 fF/μm2 needs a square electrode having a side of 8.7 μm. The capacitance will deviate from 30 fF unless the above dimensions are accurately realized. Such a deviation of the capacitance will affect the circuit operation. For example, the oscillator employing the MIM capacitor will have a frequency error that depends on a capacitance deviation. It is to be noted that current technology has a difficulty in accurate production of an electrode pattern of MIM capacitor having a side equal to or smaller than 10 μm. In other words, the MIM capacitors currently available have a large variation in capacitance.
In contrast, the interdigital capacitor is suitable for use in a millimeter wave band higher than 30 GHz, and is capable of accurately realizing a capacitance as small as tens of fF. The interdigital capacitor has a pair of comb-like electrodes, which are formed on a semiconductor substrate of silicon or semi-insulating GaAs. Electrode fingers of the comb-like electrodes are alternately arranged.
A method for adjusting the capacitance of the interdigital capacitor is described in Japanese Unexamined Patent Publication No. 6-232672. This method is now described with reference to FIG. 1. Referring to this figure, two comb-like electrodes 10 and 12 are formed on a substrate made of silicon or semi-insulating GaAs. Pairs 14 and 16 of electrode fingers for use in capacitance adjustment are provided in a path along which a surface acoustic wave travels. The capacitance can be adjusted by connecting the pair 14 of electrode fingers to the comb-like electrodes 10 and 12 by bonding wires 18 and 20.
Another frequency adjustment method is described in Japanese Unexamined Patent Publication No. 8-130433. This method employs trimming of the comb-like electrodes. The method is now described with reference to FIG. 2. Referring to this figure, two comb-like electrodes 22 and 24 are formed on the substrate. An oxide film is formed on the comb-like electrode 24 by a process of, for example, anode oxidization. The capacitance can be adjusted by trimming the comb-electrode 22 on which no oxide film is deposited. During trimming, metal of the comb-like electrode 22 is scattered and adheres to the comb-like electrode 24. However, the oxide film formed on the metal of the comb-like electrode 24 prevents scattered metal from adhering thereto. Thus, it is possible to avoid a possibility that electrode fingers or the comb-like electrodes may be short-circuited.
However, the above-mentioned two conventional techniques cannot be used in the millimeter wave band over 30 GHz.
FIG. 3 shows an interdigital capacitor usable in the millimeter wave band. In order to realize a capacitance of 0.05 pF, the comb-like electrode length L1 is approximately 60 μm, the electrode finger length L2 is approximately 50 μm, the electrode finger width W is approximately 2 μm, the finger-to-finger gap G is 2 μm, and the number N of electrode fingers is 16. It is required to adjust as a small capacitance as 0.05 pF by a step of, for example, 0.005 pF.
The adjustment method proposed in Japanese Unexamined Patent Publication No. 6-232672 is suitable for a large capacitor having a capacitance as large as 1 pF used in a relatively low frequency equal to or lower than 5 GHz. However, it is impossible to accurately adjust the capacitance as small as 0.05 pF used in the millimeter wave band equal to or higher than 30 GHz (for example, by the 0.005 pF step). This is because the bonding pads for bonding the wires 18 and 20 needs 50 μm2 at minimum, to which a parasitic capacitance of about 0.2 pF is connected.
The adjustment method proposed in Japanese Unexamined Patent Publication No. 8-130433 has a problem arising from forming the oxide film. If an element exists in the vicinity of the interdigital capacitor, the element may be damaged at the time of forming the oxide film by anode oxidization. Thus, the method cannot be suitably applied to MMIC in which the interdigital capacitor is required to be arranged close to an FET (Field Effect Transistor) or the like. In practice, the adjustable interdigital capacitor can be applied to a limited application in which the interdigital capacitor is used alone. Besides the above, there is another disadvantage in which a parasitic capacitor is connected to a pad necessary for anode oxidization.